Fossil fuels contain sulphur, which during combustion forms gaseous sulphur oxides, SOx. The amount of SOx in fuel exhausts vary according to natural differences in the sulphur content of fuels. The dominant constituent, making up more than 95% of the SOx emission from combustion of fossil fuel, is sulphur dioxide, SO2. SO2 is a toxic gas, directly harmful for both fauna and flora. A secondary effect of SO2 emission to the atmosphere is formation of sulphate aerosols and a third well recognized result of SO2 emissions is acid rain.
To meet existing and upcoming regulations on reduced sulphur oxide emissions, the use of fuel with low sulphur content is an option. However, there is limited availability of natural low sulphur fuels and the refinery process for desulphurization is costly and energy demanding. A potential sustainable alternative to the use of low sulphur fuels is removal of constituents from the exhaust gas after the combustion process.
Due to legislative requirements, certain Flue Gas Desulphurization (FGD) or scrubber techniques are being adapted from their usual land-based applications to marine applications. So-called exhaust gas scrubbers or just scrubbers seem promising for applications onboard ships. Some well-known scrubbing techniques are shortly described below.
Limestone
Wet scrubbers are well-know from especially coal fired power and cement plants where they have been a preferred solution to remove SO2 from flue gasses for decades. The flue gas is usually cleaned by circulating slurry of water and limestone under the formation of gypsum, which is collected and dewatered. A large part of the gypsum is sold and used as e.g. filler in Portland cement. SO2 removal efficiencies exceeding 98% are typical in these applications. The chemical reactions taking place can formally be written as follows:SO2(g)+CaCO3(s)+½O2(g)→CaSO4(s)+CO2(g)  (1)
Coal dust particles are usually collected in an electrostatic filter or bag filter prior to the scrubbing process, whereby contamination of the slurry and thereby the final gypsum product is avoided. Again, due to the need of particle filters and storage and handling of the powdered limestone reactant and gypsum product, usual scrubbing with limestone is considered inappropriate onboard ships.
Fresh Water with Addition of Sodium Hydroxide
In other land-based installations, aqueous sodium hydroxide is used as an alkaline neutralizing agent instead of the limestone:SO2(g)+2NaOH(aq)+½O2(g)→2Na+++SO4−−+H2O  (2)
The sodium sulfate formed is usually dissociated in the discharge water from the scrubber. From a process point of view, it is easier to use aqueous sodium hydroxide than limestone because handling of the limestone and gypsum powders are avoided. However, the applications are limited to smaller installations because of the costs of the sodium hydroxide. Sodium carbonate (Na2CO3) or sodium bicarbonate (NaHCO3) could however be used as cheaper alternatives. Another major disadvantage for usage onboard a ship is that, depending on the conditions, a large amount of freshwater will be required. However the availability of freshwater is usually limited onboard a ship.
Seawater
It is known technology to treat waste gases with seawater. The pH of surface seawater usually ranges from 8.1 to 8.9. Using this natural alkalinity to neutralize absorbed sulphur dioxide is well-known from Inert Gas Systems (IGS) onboard ships but also from several land-based installations. IGS have been supplied for more than 40 years to the tanker industry and seawater scrubbers are today an integrated part in many of these systems. With absorption in seawater, the SO2 will mainly end as bisulfite and sulfate in the water, according to the following reactions:SO2(g)SO2(aq)  (3)SO2(aq)+H2OHSO3−+H+  (4)HSO3−+O2(aq)→SO4−−+H+  (5)
The hydrogen ions then “push” to the natural carbonate balance in the water as follows:H+CO3−−HCO3−  (6)H1+HCO3−H2CO3   (7)H2CO3H2O+CO2(g)  (8)
The net result is formation of sulfate ions in the water and gaseous carbon dioxide, which is released to the atmosphere. The amount of the carbonate (CO3−−) and bicarbonate (HCO3−) ions as well as other minor anions to react with hydrogen cations determine the so-called alkalinity or buffering capacity, which in turn is a measure of the amount of SO2, which can be absorbed in the water. The obvious advantage of using seawater instead of freshwater is that no neutralization chemicals, like NaOH or Na2CO3, are required onboard the ship. The main disadvantage is that a very high water flow is required due to a limited seawater alkalinity and that seawater is relatively corrosive, whereby the costs of the scrubber construction material increases.
Adoption of Scrubbers to Marine Applications
Another challenge for the adoption of land-based scrubbers to marine applications is the changing legislative requirements and also changing conditions when a ship sails through different waters. So-called Emission Control Areas (ECA) have been established with more strict SO2 emission levels. From 2015, ships are not allowed to emit more SO2 than corresponding to 0.1% sulphur in the fuel oil within emission controlled areas. Outside emission controlled areas, the limit is 3.5% sulphur until 2020 and 0.5% sulphur after 2020. Contrary to land-based installations, this means that a scrubber must be much more efficient when a ship enters an emission controlled area as well as that adjustments must be implemented to cope with changing seawater alkalinity (in case of a seawater scrubber), seawater temperature and engine load.
Once a scrubber system has been installed onboard a ship, only limited degrees of freedom are left for adjusting the operation of the scrubber to comply with the changing legislative requirements and changing ambient conditions. An option is of course to over-dimension the size of the scrubber and the water system to thereby be able to fulfill the required efficiency even under the worst possible conditions. Examples of such possible conditions are high engine load, high fuel sulfur content, lower water alkalinity, ship in emission controlled areas and extremely low water temperature. This is however not an attractive solution as these circumstances will only occur rarely and add costs to the investment in the scrubber system and also increase the operational costs under normal sailing conditions. Normal sailing conditions typically involve e.g. 40-80% engine load, 2.3% fuel sulfur content, a seawater alkalinity on 2200 μmol/kg, and a seawater temperature between 5-15° C., In many situations, it will even be impossible to retrofit an over-dimensioned scrubber system onboard an existing ship due to very limited space available.
WO 2007/054615 describes a seawater scrubber system applicable for marine use. According to this prior art, it is suggested to reduce the amount of required seawater in the scrubber by concentrating the seawater by reverse osmosis. The fresh water also produced by the reverse osmosis can be applied for NOx reduction in the engine or for other purposes onboard the ship.
WO 2008/015487 describes a freshwater scrubber system applicable for marine use. According to this prior art, a specially designed condensational scrubber is disclosed with the aim to improve particulate matter removal efficiency and to avoid the necessity of exhaust gas reheat after the scrubber.
EP 1857169A1 also describes a freshwater scrubber system applicable for marine use. In one embodiment, a two section scrubber is suggested. The first section is for sulfur removal and the second section is for condensation, whereby the overall water consumption is reduced. In this case both sections are operated on fresh water with an addition of caustic soda. The idea of creating clean freshwater in a later condensational stage has also been suggested and disclosed in U.S. Pat. No. 5,657,630.
WO 2010/027938A2 describes a scrubber intended for cleaning flue gases from land based plants, especially metal smelting operations. The scrubber consists of two stages, a first sulfur removal stage operating on seawater and a second water condensational stage for generating freshwater.
Further, a wet scrubber was installed by Aalborg Industries A/S (Alfa Laval Aalborg A/S since May 2011) onboard a ship and has been in operation since June 2010. This scrubber is the first scrubber in the world installed after a ships main engine (21 MW MAN 2-stroke). The scrubber is called a hybrid scrubber because it has the unique possibility to operate in either a seawater mode or in a freshwater mode. The scrubber also comprises two sections. In the first section, the exhaust gas is cooled and cleaned by spraying in water in a downward flow parallel to the exhaust gas. In the second section, the exhaust gas is further cooled and cleaned by passing upwards through high surface fillings elements in a flow counter current to the downwardly sprayed water flow. A general description of Aalborg Industries proto-type scrubber system has been published on several conferences.
In the wet scrubber installed by Aalborg Industries A/S, an efficient SO2 removal has been found in the first jet sprayer section when operated in seawater mode. This can be explained by the fact that the SO2 concentration in the gas phase, which is the driving force for the absorption reaction, is relative high in the first section. In the second absorption section, a large amount of seawater is however necessary in order to remove the remaining SO2 and comply with the strictest legislation on 0.1% sulfur equivalents (SEQ). With the installation made by Alborg Industries A/S, the sulfur emission after the scrubber is usually between 0.1-0.3% SEQ when the engine is operated at full load (21 MW) and when the pump supplying seawater to both sections is working at maximum (1000 m3/h).
In freshwater mode, it is not a problem to comply with the strict 0.1% SEQ limit. A bad SO2 absorption efficiency has been found when starting up on clean freshwater but the efficiency increases significantly when the water has been re-circulated for approximately 10-30 minutes. This can be explained by the fact that also a fast reacting bicarbonate buffer builds up in the water. Beside SO2 also carbon dioxide, CO2, is absorbed from the engine exhaust gas. This CO2 will also react with the added caustic soda:CO2(g)+OH−→HCO3−  (9)
Because most of the water is circulated hack to the scrubber, the formed bicarbonate (HCO3−) will get another chance to react with SO2:SO2(g)+HCO3−→HSO3−+CO2(g)  (10)
After a certain time of operation, a bicarbonate buffer will thereby build up in the thus circulated water. This bicarbonate buffer will cause the liquid to maintain a high pH during the entire SO2 absorption process and thereby improve the overall SO2 absorption efficiency.
In practice it has been found difficult to suddenly switch over from seawater mode to freshwater mode because some seawater will enter the freshwater system or because contaminated freshwater will be discharged to the sea.